The present invention relates to an apparatus and method for drilling composite materials, particularly printed wiring boards.
Printed wiring boards provide a means for mounting and connecting circuit components, e.g. integrated circuits, in electronic devices. Such a board may include an insulating substrate which is copper clad, with the copper cladding selectively etched to provide the desired circuit paths. The materials from which the substrates of printed wiring boards are formed are preferably of low heat capacities in order to avoid the conductance of heat and electricity which could adversely effect the proper functioning of the circuits connected thereto. Thus, printed wiring board (PWB) substrates are desirably formed of carefully selected materials, for example, glass reinforced polymers. In order to make proper electrical connections on the PWB, the PWB is formed as a laminate comprising layers of copper on the surface or surfaces of the substrate material. The copper surface or surfaces are then etched leaving a desired pattern of copper electrical connectors. Since the PWB serves as a mounting board for circuit components having protruding leads, it is usually necessary to drill holes in the PWB to receive the leads of the circuit components.
Known methods of drilling holes in printed wiring boards include using a mechanical drill utilizing N/C controlled drilling machines such as ADVANCED CONTROLS TRUDRILL MODEL 95 or an Excellon Model Mark IV. These types of drills use drill bits formed from tungsten carbide. In order to avoid drill bit breakage, maintain the consistency of the drilled hole, and avoid excessive damage to the portion of the PWB immediately adjacent to the drilled hole, the chip load (feed rate) has to be kept below about 0.5 mils/revolution and the number of hits per drill bit to about 500. It has been found that mechanical drilling can be expensive since tungsten carbide drill bits usually cost between $4.00 and $12.00 each, making the total drill bit cost alone about $60.00 for the mechanical drilling of very high density micromodules having 2500 holes. Furthermore, it has also been found that mechanical drilling is not suitable for drilling certain substrates, including quartz or kevlar reinforced polymers as well as homogeneous fused quartz substrates.
Another known approach, though not as widely practiced as mechanical drilling, is to use a single laser such as a neodimium YAG (Ytrium Aluminum Garnet) laser or carbon dioxide laser which are normally used in a pulsed mode. The single laser drilling method has been found to be satisfactory for PWBs having low laminate thicknesses of approximately 10 to 20 mils with no copper or low copper thicknesses, e.g. 0.0007 mils or lower. The single laser drilling approach, however, has a limited ability to penetrate through copper/polymer composites especially where the copper thickness exceeds 0.0007 mils. In order to drill through greater thicknesses, it has been found necessary to increase the power of the single laser. However, increasing the power adversely affects the area around the laser drilled holes possibly resulting in delamination. Thus, there is a need for an improved and less expensive manner of drilling holes in PWBs.